1. Field of the Invention
The invention relates to a method for mechanically joining concrete-reinforcing rods, to a reinforcing rod, allowing the application of said method as well as to a mechanical joint of reinforcing rods thus produced. The invention is applicable in particular in the construction of concrete building components or concrete structures.
2. Discussion of Background and Relevant Information
Currently, such reinforcing rods are connected by way of joints having the function of transmitting the tensile stress; in addition, the joint must be easy to set in place and be of a low cost. Various solutions have been proposed by constructors to bring about the mechanical joining of reinforcing rods.
There exists first of all the overlap joint system. This method has several drawbacks. In particular, it is necessary to leave pending a relatively big length of the reinforcing rod, up to two meters, for example, in order to subsequently produce the joint, which is troublesome and difficult and often even impossible to fold back by reason of the large diameters encountered.
Another proposed method consists in utilizing a mechanical joint. There is known for example the system of conical threading in which the extremities of the rods to be joined are machined to have the shape of a conical thread, by means of which they can be inserted and screwed into a connector to be embedded in a block of concrete.
This solution suffers from numerous drawbacks. In particular, the conical threading is effected on a solid bar at the nominal diameter of the reinforcing rod. The machining of the thread diminishes locally the cross-section of the bar which then corresponds substantially to the cross-section at the bottom of the thread.
During tensile tests, the rupture of the reinforcing rod always occurs at its extremity in the threaded zone. Consequently, it cannot be doubted that this method of mechanical jointing weakens the reinforcing round, which must be overdimensioned accordingly to take into account the local weakness in the threaded region.
Nor does this system allow the use of a simple connecting sleeve with right-hand and left-hand thread to constitute the joint with adjustment of tension. It is necessary to employ several parts to constitute a stack and allow for adjustment of length.
Lastly, the tightening of the sleeve on the conical thread must be effected with a preset torque, which has to be monitored. This operation is not easy to perform on a construction site, but it is nevertheless indispensable for safety reasons. If the tightening torque is not achieved, there are risks of dislocation and a total absence of resistance to traction.
From an economic point of view, this is an expensive solution, because the machining of the sleeve is a complex operation and, in particular, the thread must be cut in two stages.
In conclusion, this technique necessitates an overdimensioning of the diameters of the reinforcing rods of the order of 20%, in order to withstand the stresses which concentrate at the threaded extremities of the resulting in high costs.
Another mechanical joint has also been used. This consists in a crimping of the extremities of the reinforcing rods to be joined. To this end, there is employed a socket into which are inserted the two extremities of the reinforcing rods. The socket is then crimped on the rods with the aid of a jack and a press.
This technique is fraught with high risks of slippage owing to the crimping which is far from easy to achieve and difficult to control. This defect considerably reduces the mechanical strength of the joint. On a construction site, it is often difficult to position a press level with the socket to be crimped. Also, the use of a press is costly.
Further, with regard to the regulations governing the use of such mechanical joints of reinforcing rods it is of course prescribed that the latter must be able to withstand ultimate rupture stress and certain Countries, especially the Anglo-Saxon Countries, impose very rigorous slippage-control standards.
In Great Britain, for example, Standard BS-81 10: part 1; 1985-3.12.8.16.2 specifies that reinforcing rods assembled by means of a connecting sleeve must be able to withstand a tensile test in which the rods are subjected to a stress corresponding to 60% of the elastic limit, following which the permanent elongation may not exceed 0.1 mm.
These standards are even more rigorous in some other countries. For example, in the United States, the stress applied corresponds to 80% of the elastic limit. Similar tests are also applied in the nuclear industry.
These tests, when carried out on a site, are difficult to put into effect, requiring the use on the site of torque wrenches, which increases the cost of the finished joint.
Moreover, if the machining of the different components has not been carried out with precision, it may happen that during subsequent testing the mechanical joint does not satisfy these standard specifications. It is then necessary to start all over again, which is affects production costs. On the other hand, precision fabrication requires a highly skilled workforce and special attention to detail of such an order that the solution ceases to be an economically viable one.